72 VAV Controller—Variable Air Volume Controller
The stroke time of an incremental valve actuator is important because the
controller uses it to position the actuator in reference to an analog 0-100%
open position command. If the stroke time is grossly inaccurate, the
controller drives the actuator under an incorrect assumption of total stroke
time and causes inaccurate positioning, particularly at the near closed or
near full open positions. If possible, measure the actual stroke time of the
incremental actuator with water flow through the valve. Some valves have
1/2, 5/16, or 3/4 inch strokes, which require different stroke times.
If you do not measure the actual stroke time, you at least need to know the
approximate stroke time of the valve and enter that in the HVAC PRO for
Windows configuration tool. Conservatively, you can add up to 20% to the
nominal time. Always add to the stroke time, never subtract. The
controller modulates the valve based on a calculated stroke time of 150%
of the stroke time you specify. This is done to compensate for variations in
water system pressure, actuator wear, and manufacturing tolerances.
The VAV controller has a built-in overdrive algorithm that drives the
actuator closed for an additional 50% of the entered stroke time when a
0 or 100% open command is issued. This ensures that the valve is fully
closed or open. This fully opens or closes the valve, but does not correct
the mid-stroke position errors due to a grossly incorrect valve stroke time.
Refer to the following table for approximate stroke times.
Table 32: Approximate Stroke Times
Incremental Valve
Actuators
Stroke Time
VA-8050-1
1/2 in. stroke 65 sec (1.1 min)
VA-8051-1
1/2 in. stroke 65 sec (1.1 min)
VA-8050-1
3/4 in. stroke 90 sec (1.5 min)
VA-8051-1
3/4 in. stroke 90 sec (1.5 min)
VA-8020-1
5/16 in. stroke 90 sec (1.5 min)
VA-8050-1
5/16 in. stroke 45 sec (0.75 min)
VA-8051-1
5/l6 in. stroke 45 sec (0.75 min)
Note: There may be no correlation between valve position and controller
output (0 to 100%). If you require correlation, use proportional
control or actuators with position feedback.
84F85F86FIncremental
Valve Actuator
Stroke Time
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